Ultrasound reflection imaging is a powerful tool in diagnostic medicine. Transducer array systems are usually used in medical imaging, particularly for real-time applications. One simple form of a transducer array is the collimated imaging array. It consists of a row of individual transducer elements disposed side-by-side. The individual elements (or small groups of elements) are actuated in sequence to transmit a beam of ultrasound energy. Echo data is received on the same elements for each B-mode line in a display. This produces a rectangular format image. Such arrays have been successfully used for small organ imaging applications, such as the eyes, where relatively short wavelengths can be used. When imaging applications which require deeper penetration attenuation forces the use of larger wavelengths. Collimated imaging arrays are not used for this purpose, since diffraction causes the beam to diverge and degrades resolution.
The performance of transducer arrays has been improved by the use of controlled delay elements which produce electronic focusing. In these systems, each element receives signals from many points in the image. Signals which are received by the individual elements are appropriately delayed and summed to produce individual B-lines which form an image. Dynamic focusing may also be utilized for reception of signals. The focal length of the array is varied electronically as a pulse propagates through the body. This ensures optimum lateral resolution at all depths.
In certain applications, such as cardiac imaging, only a small window is available for the array. In these applications, a small array may be used with electronic deflection, as well as focusing, to produce a sector-scan image. These systems are referred to as "Phased Arrays," in order to distinguish them from "Linear Arrays," which project beams perpendicular to the face of the transducer elements and use only electronic focusing. The principles of the present invention apply both to Linear Arrays and Phased Arrays.
The point spread function (PSF) of a linear imaging system is the image it produces for a point object. A commonly used criteria for measuring the resolution of an imaging system is the Rayleigh criteria which characterizes the ability of the system to differentiate two closely spaced point targets. The width of the main lobe of the PSF is a measure of this ability. This criteria, although important, is not ideal for ultrasound medical imaging applications where human tissue acts as a diffusely reflecting structure. It is often important to be able to differentiate subtle tissue structures in the presence of strong reflectors. This ability is limited by the sidelobes of the PSF. It is, therefore, of great interest to develop systems with small sidelobe levels.
The conventional technique for reducing the sidelobe levels of a transducer array is "Aperture Apodization". Aperture weighting functions, such as Gaussian or Hanning functions, are applied to signals on the array elements. Such techniques are described, for example, in Peterson et al Quantitative Evaluation of Real-Time Synthetic Aperture Acoustic Images, in: A Review of Progress in Quantitative Nondestructive Evaluation (Plenum Press, New York, Vol. 1, 1982, pages 767-776) and in 't Hoen Aperture Apodization to Reduce the Off-axis Intensity of the Pulsed-mode Directivity Function of Linear Arrays, Ultrasonics, September 1982, pages 231-236, which are incorporated herein, by reference, as background material. Using these prior art techniques, the sidelobe level can be reduced at the expense of some loss in main lobe width. This trade-off is a fundamental one which is also encountered in spectral analysis and antenna design. When energy under the sidelobes is is reduced, more energy is introduced under the main lobe.